1. Field of the Invention
Embodiments of the invention described herein pertain to the field of submersible pump assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable a motor shroud for an electric submersible pump.
2. Description of the Related Art
Submersible pump assemblies are used to artificially lift fluid to the surface in deep wells such as oil or water wells. A typical electric submersible pump (ESP) assembly consists, from bottom to top, of an electric motor, seal section, pump intake and centrifugal pump, which are all connected together with shafts. The electric motor supplies torque to the shafts, which provides power to the centrifugal pump. The electric motor is generally a two-pole, three-phase, squirrel cage induction design connected to a power source located at the surface of the well using a motor lead cable. The entire assembly is placed into the well inside a casing, which casing separates the submersible pump assembly from the well formation. Perforations in the casing allow well fluid to enter the casing. These perforations are generally below the motor and are advantageous for cooling the motor when the pump is in operation, as fluid is drawn passed the outside of the motor as it makes it way from the perforations up to the pump intake.
One challenge to economic and efficient ESP operation is pumping gas-laden fluid. When pumping gas-laden fluid, the gas may separate from the other fluid due to the pressure differential created when the pump is in operation. If there is a sufficiently high gas volume fraction, typically about 10% or more, the pump may experience a decrease in efficiency and decrease in capacity or head (slipping). If gas continues to accumulate on the suction side of the impeller it may entirely block the passage of other fluid through the centrifugal pump. When this occurs the pump is said to be “gas locked” since proper operation of the pump is impeded by the accumulation of gas. As a result, careful attention to gas management in submersible pump systems is needed in order to improve the production of gas-laden fluid from subsurface formations.
Conventionally in wells with gas-laden fluid, perforations in the well assembly casing are sometimes placed above the pump intake, rather than below the motor. In such instances, a shroud is placed around the pump base, intake, and motor lead cable, which shroud includes a jacket (a length of tubing) that extends below the motor, in order to prevent fluid from entering through the perforations and proceeding directly to the pump intake. Instead, once the fluid enters the perforations the liquid is forced downward in between the shroud and casing. In the process, a portion of the gas breaks out of the laden fluid prior to entry into the pump and naturally rises up the open casing annulus to the surface, instead of down to the bottom of the shroud with the liquid. Once the liquid reaches the end of the shroud jacket it makes a 180 degree turn, is forced upward, and enters the inside of the shroud by the motor. This configuration still maintains the advantageous motor cooling, as the well fluid will now pass over the outside of the motor as it makes its way into the pump via the intake, whilst beneficially separating some gas from the laden fluid.
A drawback to the use of a shroud is that conventional shrouds are prone to leaks. If well fluid were to leak directly into the pump, the fluid would bypass the motor, which would be at risk of overheating or failure due to the lack of cool, fresh flowing fluid passing by during operation. Those portions of the shroud surrounding the motor lead cable and intake section are particularly prone to leakage.
One conventional approach to protect against leaks in the shroud is the addition of layered and slotted rubber material that squeezes around the motor lead cable and intake section to provide a positive seal. However, handling and fitting up the rubber material is difficult and extremely time consuming because of the tight fitting clearances, and if the weather is cold such as below 32° F., the cold weather makes it difficult to squeeze the rubber in the fashion necessary to create the positive seal. Even if the rubber material is installed, it is limited in surface area. Another approach has been to use tape to fill voids in the shroud, but the tape is also temperamental under temperature extremes, such as below 32° F. or above 100° F.
It would be an advantage for motor shrouds to be resistant to leaks, and expeditious and simple to install at the well site despite extreme weather conditions. Therefore, there is a need for a motor shroud for electric submersible pumps.